First flight of the Orbital Sciences Corp. Cygnus commercial cargo carrier to the International Space Station will slip at least a month because of a delay of three to four weeks while a surplus Soviet-era engine is replaced on its Antares launch vehicle.

With the delay, Orbital says it will be ready to launch the first complete Antares/Cygnus stack from Wallops Island, Va., in early August instead of late June or early July as originally hoped. However, a potential conflict with the arrival of another Japanese H-II Transfer Vehicle (HTV) could slip the first Cygnus berthing with the ISS into September, Orbital said.

“If the HTV schedule slips, Orbital expects to be ready to go in August,” the company said in a website update. “If the HTV holds its schedule, Orbital’s Demonstration Mission could be planned for September.”

The “Demonstration Mission” will mark the final milestone under Orbital’s $288 million Space Act Agreement to develop a commercial resupply route to the space station under the Commercial Orbital Transportation Services (COTS) seed-money effort. If it is successful, the Dulles, Va.-based company can begin delivering cargo to the station under its eight-flight, $1.9 billion Commercial Resupply Services (CRS) contract with NASA.

SpaceX of Hawthorne, Calif., which also received COTS funding and holds a follow-on CRS contract, already has used its Falcon 9/Dragon combination to deliver two loads of cargo to the ISS.

Orbital flew the Antares for the first time on April 21, demonstrating that the medium-lift rocket could fly from its new state-owned pad at the Mid-Atlantic Regional Seaport on Wallops Island, and separate a simulated Cygnus vehicle.

At the time of that mission the Cygnus to be used in the upcoming Demonstration Flight already had been loaded and fueled at NASA’s Wallops Flight Facility, in preparation for the late June launch date. However, Orbital said it needed to replace one of the two Aerojet AJ26 kerosene-fueled engines in its first stage”to further inspect and confirm a seal is functioning properly,” according to the company website.

The AJ26 is a Soviet-era NK-33 engine that has been refurbished by Aerojet. The engine’s configuration makes internal inspection difficult, and one of the AJ26s in Aerojet’s inventory was badly damaged in a test-stand fire at Stennis Space Center when it leaked fuel during a hot-fire acceptance test for the Antares in June 2011.

Orbital said the two engines for the first CRS cargo flight of Antares/Cygnus, tentatively set for the fourth quarter of this year, “have been fully tested and are already at Wallops.”

Data analysis from the April 21 inaugural Antares flight found its propulsion, guidance/navigation, attitude control, other flight systems and ground control systems “all performed as designed,” Orbital reported Monday.

Instruments on the Cygnus mass simulator the rocket sent into orbit “confirmed Orbital’s engineering models that predicted a benign launch environment for Cygnus and other future satellite payloads in terms of accelerations, vibrations, acoustics, thermal and other measurements captured during the flight,” Orbital said.

The Antares uses twin AJ26s to power a kerosene-fueled first stage built by Yuzhnoye of the Ukraine, with an ATK Castor 30 as the solid-fuel upper stage. The Cygnus cargo capsule is built by Thales Alenia Space in Turin, Italy, while Orbital builds the Cygnus service module at its satellite manufacturing facility in Dulles.